The overall purpose of this continuing project is to understand the contributions of mechanisms at various levels of visual processing to color vision, with particular interest in higher level mechanisms, that is, those beyond the conventional second stage mechanisms. A central aspect of the work is the coordination of investigations in psychophysics done at New York University and electrophysiology done in a continuing collaboration with Prof. Peter Lennie at the University of Rochester. The main aims are: 1. To study color discrimination under conditions of constant adaptation over an extended range of chromaticities and luminances and in detail over theoretically important regions. 2. To further test the hypothesis that photon noise plays a role in determining the detectability of increments on backgrounds. 3. To advance knowledge about the relative number and distribution of cone receptors by measuring the color appearance of small monochromatic stimuli imaged with the aid of adaptive optics. 4. To determine whether there are privileged directions in color space along which observers more readily abstract color appearance. 5. To continue collaborative electrophysiological experiments relating to significant issues in color vision. 6. To attempt to resolve the conflict between estimates of the relative sensitivity of the L and M cones based on two lines of evidence.